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"Questions on applications or application features that are no longer available are off-topic for Web Applications as no one will ever be able to make use of the answers again." – Al E., Alex, Eight Days of Malaise, jonsca

at the bottom of the feed item there is a list of commands and one of them is "Keep Unread"

or you can just press the m key to toggle the read/unread status of that item

In regard to items not being available to mark unread after 30 days (i did not know this)...one approach to get around this is to use a service such as instapaper to save articles for later reading.. and to go even further you can grab an RSS feed for your instapaper account and load that as a feed into Google Reader so you can see all of your "unread" items in instapaper in one feed.. this is what I do

I only get the Keep Unread option on posts from the last two days, so I think the rationale is that you use this when you have a quick look at a recent post and decide to read it later, and if you want to keep track of older posts, you can star them.

Hmm, could it be Google is A-B-testing on us? I definitely still have the "Keep unread" option on some really old items...

What I do notice however is that there appears to be no way back once you've hit the "Mark all as read" button: Any item that was marked as read in this manner (rather than by actively opening and reading it) cannot be brought back into unread state ever.

This is also a possible scenario as it happens to me each time. If I just browse through a few items of one feed and decide to mark them ALL as read, the "keep unread" option will then disappear for those items in the feed.